1. Technical Field
The present invention relates to alloy nano-particles having high coercive force, more precisely to those which have a low transformation point and hardly aggregate and which can form a flat magnetic film having high coercive force.
2. Background Art
It is important to reduce the particle size of the magnetic matter in a magnetic layer for increasing the magnetic recording density of the layer. For example, in magnetic recording media that are widely used for video tapes, computer tapes, discs and others, ferromagnetic particles having a smaller particle size are better for noise reduction than larger ones having the same mass.
Recently, CuAu-type or Cu3Au-type hard-magnetic ordered alloys have been developed as materials that are hopeful for increasing the magnetic recording density in recoding media. Such CuAu-type and Cu3Au-type alloys all have high magnetocrystalline anisotropy owing to the strain that occurs therein when they are ordered, and they exhibit good hard magnetism even when their particle size is reduced to a level of nano-particles.
Methods for producing nano-particles capable of forming those CuAu-type or Cu3Au-type alloys are known, and based on a mode of precipitation, they are grouped into an alcohol reduction method of using a primary alcohol; a polyol reduction method of using a secondary alcohol, a tertiary alcohol, or a polyalcohol such as diol or triol; a thermal decomposition method; an ultrasonic decomposition method; and a super-reduction method of using a strong reducing agent. Based on a mode of reaction system, the methods are grouped into a polymer-in-site method, a high-boiling-point solvent method, a normal micellation method, and a reversed micellation method.
In the alcohol reduction method, the reducing power of the alcohol used is weak, and therefore the method is problematic in that, when a noble metal and a base metal are simultaneously reduced, core/shell structured particles are often formed and uniform alloys are difficult to produce. The polyol reduction method and the thermal decomposition method indispensably require high-temperature reaction and are therefore problematic in that they lack production latitude in point of the production costs, etc. The ultrasonic decomposition method and the super-reduction method are relatively simple methods, in which, however, aggregates and precipitates are readily formed and therefore monodispersed fine nano-particles are difficult to produce if the reaction system is not specifically designed for producing them.
As a combined system of the alcohol reduction method and the polymer-in-site method, known is an ethanol reduction method to be effected in polyvinylpyrrolidone. In this, however, the polymer amount after the alloy production is extremely large, and therefore the method is problematic in that it is difficult to reduce the polymer amount to the necessary one. As a combined system of the polyol reduction method, the thermal decomposition method and the high-boiling-point solvent method, for example, known are the methods described in JP-A 2000-54012 and U.S. Pat. No. 6,254,662. In these methods, however, highly-toxic substances must be used and they are extremely dangerous, and, in addition, the reaction must be effected in an inert gas at a high temperature of around 300° C. Therefore, these methods are problematic in that the constitution of the apparatus for them is complicated and they lack production latitude. A combined system of the super-reduction method and the polymer-in-site method, and a combined system of the super-reduction method and the reversed micellation method are general methods, for which, however, detailed conditions are not as yet found for obtaining metal nano-particles that have an intended composition and an intended particle size.
The nano-particles produced according to the above-mentioned methods have a face-centered cubic crystal structure. Face-centered cubic crystals are generally soft magnetic or paramagnetic, and they are unsuitable for magnetic recording media. Therefore, for obtaining hard-magnetic ordered alloys that have a coercive force of at least 110 kA/m (1382 Oe) necessary for magnetic recording media, the nano-particles must be annealed at a temperature not lower than the transformation point thereof at which their disordered phase is transformed into an ordered phase.
However, when the nano-particles produced according to the above-mentioned methods are applied onto a support and annealed to fabricate magnetic recording media, they readily aggregate together, and therefore they are problematic in that their coatability is poor and their magnetic properties are not good. In addition, they are still problematic in that, even when they are annealed, their phase could not be completely transformed into an ordered phase and they could not have the intended hard-magnetic property. Further, the transformation point of the nano-particles in the magnetic recoding media is generally at least 500° C. and is high, and ordinary organic supports are not resistant to such high temperatures. To that effect, there is still another problem with the nano-particles in that it is difficult to form a magnetic layer on such an organic support by applying the nano-particles onto it followed by annealing them thereon.
The present invention has been made for solving the above-mentioned problems, and an object of the invention is to provide nano-particles having high coercive force, especially those which have a low transformation point and hardly aggregate and which can form a flat magnetic film.